QT3 by default does not have support for the scim-bridge drivers. Instead, you will need a patched version of QT3 which has been compiled with immodule support.

Currently, this version, qt-im-3.3.8.pet, is available here.
The scim-bridge driver is included in the package.

This package installs to /usr/lib/qt-im-3.3.8. Therefore, it will not overwrite or get mixed up with any other version of QT3 that might be on your computer. You will, though, need to adjust your /etc/profile to tell the system to use this version of QT3. It was originally intended to have an install script do this, but a bug in Puppy has prevented this for the moment.

QT plugins cannot be "mixed and matched". You can't take a plugin from another compilation, even if it's the same version number, and expect it to work. The safest thing is to make sure that they've been compiled against the specific version of QT that you are going to use. Alternatively, compile the plugins yourself.

If you are installing QT3 or QT4 which has been compiled in another system, the installation location must match, symlinks and all. For example, if QT3 was compiled to /usr/lib/qt3, which was a symlink of /mnt/home/qt-x11-free-3.3.8, you need to set it up that way in the new system. Otherwise, it can't find header files and immodules won't work.